A method for measuring powder distribution on the basis of the granular composition of particles by photoelectric scanning and a device for the implementation of that method are known (see British Patent No. 1069680, Aug. 21, 1964) in which particle size distribution in a powder sample being analyzed is directly measured. This method is carried out in a scanning mode wherewith a narrow beam of light is moved in the direction of the force of gravity. In the case of a homogeneous suspension of particles, the particle size distribution curve may be obtained by measuring the relationship between particle concentration and depth. It is assumed that the lateral thickness of the light beam is negligibly small and the sedimentation of the suspension particles takes place in a stationary Stoke's mode. The method provides a calculation algorithm within the limits of these assumptions.
A narrow beam of light having a negligible cross-sectional thickness is used in the aforementioned method. However, the citerion defining fulfillment of this condition is not determined quantitatively, and therefore, there is no exact assessment of the measurement error due to the existence of a finite cross sectional area in real light beams.
In the mode of continuous scanning in the direction of the force of gravity there will always be a loss of information on the quantity of small-size particles (r.sub.min), since they lag behind the scanning beam as they descent. Furthermore, measuring time increases when scanning is conducted in this direction.
In this method the moment at which sedimentation begins is taken as the starting point, following which after an arbitrary time period the source of light begins to move from the surface of the suspension downwards. Since the length of the pause after sedimentation begins is not fixed, the part of the powder consisting of larger particles with maximum radius is not fully registered.
A photoelectric method of sedimentation analysis of dispersion systems of a homogeneous substance is also known, wherein a beam of rays is passed through an unloaded cell and the initial value of the photocurrent created by a beam which is not scattered is measured. Then the cell is loaded with a suspension of uniformly dispersed particles of powder. The value of the photocurrent which is created by the non-scattered part of the beam is measured during the process of sedimentation. The relative optical density D(t) of the suspension of dispersed particles of powder is measured according to the equation EQU D(t)=ln I.sub.o /I.sub.t
where I.sub.o is the photocurrent created by the beam of rays passing through the unloaded cell, I.sub.t is the photocurrent at the current moment of time t. Then, using Stoke's equation .upsilon.=.alpha.r.sup.2, the values of the radii r of the particles descending in the suspension are determined, .upsilon. being the velocity of the descending particles (see, for example, P. A. Kouzov "Basics of Dispersion Analysis of Industrial Dust and Pulverized Materials," Leningrad, Khimaya, 1971, pp. 169-180).
Wherein, the relative optical density of the suspension is proportional to the total surface S.sub.o.sup.r of particles of the dispersive phase, the radius of which is between zero and r EQU S.sub.o.sup.r =K ln I.sub.o /I.sub.t
where K is a constant characterizing the optical properties of the measuring system which do not depend on the process of sedimentation or on the properties of the photocurrent registering system, the surface S.sub.r.sup.r, of all particles whose radius is within the range of from r' to r is determined through S.sub.o.sup.r. Then, using the equation ##EQU1## the mass of the fractions whose particles have radii within the range of from r' to r is determined, I.sub.r, and I.sub.r being photocurrent values registered at the moments related to radii r' and r by Stoke's Law of sedimentation, .rho. is the density of particle material, r.sub.m is the mean arithmetic value of radii r' and r, the coefficient K is experimentally determined.
Furthermore, the aforementioned method provides for a beam of parallel light rays formed with a special "condenser-diaphragm" system being directed to the measuring cell in a direction perpendicular to its sides. The light scattered by the suspension passes through a second diaphragm, is focussed by means of a second condenser onto a photocell, and is converted into electric current which is registered by an instrument. It should be noted that the depth at which the beam passes relative to the suspension surface does not change in time.
Only the integral characteristics of dispersion are directly measured in the aforementioned method. Data of a differential character cannot be obtained without further processing. Both procedures require considerable time.
Systems forming collimated beams of parallel light rays, making the technical realization of the method more complicated, are used. The desired to create very narrow beams simultaneously makes the measurement results sensitive to fluctuations in the suspension, which distort Stoke's model of gravitational sedimentation.
Serious problems arise when consideration is given to the use of the method with particles which approach Brownian motion behavior by size and character of movement. The reason is that there is no quantitative assessment of the effect of movement of particles of this type on measurement error.